This invention relates to electromechanical actuators of the screw driven variety. The prior art discloses a number of electrically driven screw operated jacks, Roos U.S. Pat. No. 1,324,851 and Hott U.S. Pat. No. 1,811,629 being typical. An electric motor is used to drive a lead screw which is threaded into a tubular piston-like member which is prevented from rotating when the screw is turned. Thus, rotation of the lead screw imparts a translational motion to the piston-like member. Devices based on the same principles of operation but adapted to a wider variety of installations have been developed. In principle, these so-called actuators could be used wherever a fluid operated cylinder and piston could be used. An electromechanical system is often preferable since it is relatively self-contained and does not require hoses, pumps, or messy fluids. Without fluid seals to maintain, the electromechanical devices are more reliable.
A major area of concern with electromechanical actuators is the provision of means for detecting the travel of the piston at its limits and shutting off the motor. Failure to do this can result in mechanical jamming of the device and/or burning out of the motor as a result of its being stalled.
A common method of shutting the motor off at the end of the piston's travel uses a limit switch inside each end of the actuator's outer housing. The use of the limit switches in this manner presents several difficulties. The switches' inherent unreliability is enhanced by the fact that the switches are within the physical confines of the actuator and are thus subjected to extremes in ambient conditions and applied stresses due to the rugged environment in which the actuator is likely to be used. The failure of a switch can lead to jamming of the device as well as motor burn out. Even if no additional damage to the actuator is caused by the failure of the switch, the actuator is rendered inoperable and must be disassembled, possibly requiring actual removal from its environment.
A further difficulty with locating limit switches at each end of the outer housing is that both ends of the housing are required to possess relatively complicated structure. This makes it impossible to provide an actuator whose length can be easily varied by simply cutting the housing and other associated parts, since both ends of the original housing are critical to the proper functioning of the actuator. In addition, the structure necessary for the switches provides a constraint which makes it difficult or impossible to tailor the external dimensions of the actuator to a desired configuration such as making them conform to standardized dimensions for pneumatic and hydraulic cylinders. Therefore, it is very difficult to replace existing fluid operated devices by electromechanical actuators to achieve the benefits of simplicity and increased reliability.
A number of prior art actuators do not actually prevent the drive nut from rotating, but rather rely on the load itself to prevent rotation relative to the outer housing. Under extreme loading conditions such as might occur at end of travel or under overloading of the device, an increased tendency of the drive nut and piston to rotate occurs. A conventional way of handling this problem is to provide an overload clutch between the load attachment point and the piston. This had the disadvantage of allowing the load point and the attachment point to rotate relative to one another.
Alternate methods of guiding the piston include using a piston of square cross section or a keyway and pin configuration. Sealing the device to exclude dirt from the inside is difficult with both these arrangements.